1. Field of the Invention
The invention relates to a composite arch structure of the type comprising an arched liner and compacted fill material thereagainst to form a soil arch, and more particularly to a method and means for providing controlled settling of the arched liner resulting in an improved soil arch with greater load supporting capabilities.
2. Description of the Prior Art
The teachings of the present invention are applicable to any type of arch wherein load transfer to some form of abutment means is contemplated. For purposes of an exemplary showing, the invention will be described with respect to its application to composite arch structures of the type comprising an arched liner with compacted fill material thereagainst to form a soil arch thereabout. Such structures are used, for example, as highway or railway culverts, bridges, protective underground structures, snow sheds and the like. While the arched liner may be made of any appropriate material including concrete or plastic, again for purposes of an exemplary showing the invention will be described in terms of the use of liners made up of relatively thin, flexible, arcuately curved, corrugated metallic plates. The edges of the arched liner are supported on footers and the arched liner itself, when viewed in end elevation, may have any appropriate curved shape such as a truncated ellipse, parabola, hyperbola or a circle.
U.S. Pat. No. 3,508,406teaches a number of embodiments of composite arch structures of the type to which the present invention is directed. When the arched liner is made of relatively thin, arcuately curved, corrugated metallic plates or the like, it will be understood that the liner will be relatively flexible and will have sufficient strength to permit the liner to be properly installed. However, the strength of the liner is not at all capable of supporting the superimposed load of the completed composite arch structure. The final strength of the structure resides primarily in the compacted soil arch about the liner. When the span of the liner is relatively large, it may be provided with longitudinally extending buttress means affixed to the liner. The buttress means may be performed concrete buttresses or may be poured in place during the construction of the composite arch, as is taught in the above mentioned U.S. Pat. No. 3,508,406. Sand or gravel filled bins similarly affixed to the liner accomplish the same purpose.
Heretofore, footers for the liner of a composite arch of the type contemplated were prepared by first digging a trench for each footer having a width for the size footer desired, based upon the expected load and the supporting characteristics of the soil beneath the footer. The trench for each footer ran the entire length of the structure. Concrete was then poured in each trench to form the desired footers.
If the footers were to extend above the level of the ground, forms were used for those portions of the footers above ground. A number of methods were used to attach the edgemost plates of the arched liner to the footers. One common method entailed the use of L-shaped channels affixed to and extending the length of the concrete footers. The long wall of the L-shaped channel of each footer was provided with holes, so spaced as to align with corresponding holes in the edgemost plates of the arched liner to permit these plates to be bolted to their respective L-shaped channels. Frequently, these bolt holes in the long walls of the L-shaped channels were in the form of horizontal slots so as to permit some leeway in fitting the edgemost liner plates to their respective L-shaped channels. The channels were of such width as to accept the corrugations of the liner plates. The bottom of each channel element carried a plurality of anchoring lugs embedded in the concrete footer prior to the hardening thereof.
The basic cross-sectional configuration of the footers varied with the size and type of structure being built, the angle of entry of the arched liner edges into their respective channels and like considerations. The footers, for example, could constitute an integral part of a concrete slab or roadway extending therebetween.
As will be understood by one skilled in the art, the fill material about the liner will tend to shift downwardly or settle. This settling is the result of a number of different factors. During the placement of the back fill about the liner, compaction will artificially produce a volume change in the fill material by rolling, vibration, tamping and other momentary load applications. With the passage of time, a volume change will occur in the fill material by virtue of consolidation. A static external load will produce a volume change by compression. Finally, shrinkage may produce a volume change in the fill material by virtue of capillary stresses during drying of the fill material. As used herein and in the claims hereafter, the terms "settle" and "settling", as applied to the fill material, should be construed broadly enough to encompass any one, any combination or all of the above noted types of volume change and downward shifting of the fill material.
The present invention is based upon the discovery that if yielding footer means are used, permitting controlled settling or peripheral shortening of the liner, the load upon the liner is relieved and a stronger soil arch with improved load supporting capabilities results. It will further be understood by the skilled worker in the art that controlled settling of the liner will result in a diminishing of the cross-sectional area encompassed by the liner and the bottom of the tunnel defined thereby. In this sense, the settling of the liner results in a peripheral shortening thereof.
While in most such structures the amount of settling falls within a matter of inches or less, the specific amount and the specific rate of settling are not the primary considerations. The primary factor, among many, is the relative rate of settling between the liner and the fill material. The settling of the liner should be so controlled as to be equal to or greater than the settling of the fill material.